it is not up to you, it is up to the users mail client.  If they use one of the free email account then usually they only do text mail because of spam content. If they have a third party client like thunderbird they can set their preference in that. 

The only way you can choose in oxwall is via mass mail, thats the only option that i know of. 

Hi there,

HTML email go through correctly, But when i send email as text the written message wont be shown by my recievers (they recieve email with empty body)

please advice.

who are the mail clients receiving the emails?

i tried on two diffrent scenarioes and got same result:

smtp on: no message (body) 

smtp off (using php mailer from contactus plugin): agian no message (body)

i receive an empty email with the right subject and website logo.

have you tried this on the demo site, is it the same?   Also what i was asking is who are the email clients, hotmail, msn, gmail?